The first vertebrate semaphorin protein characterized, collapsin-1/semaphorin 3A (Sema3A), has potent neuronal growth cone collapsing activity, and the independent discovery by us and others that neuropilin-1 (Npn-1) is a Sema3A receptor revealed the first step by which semaphorin signals are propagated. Npns are obligate ligand binding subunits, and members of the Plexin family are signal transducing subunits of the Class 3 Semaphorin receptors. Sema3F signals predominantly through a Npn-2/PlexinA3 holoreceptor complex, and our work shows that Sema3F-Npn-2/PlexinA3 signaling also controls dendritic spine morphogenesis and the development and function of excitatory synaptic connectivity in the murine hippocampus and cortex. We recently found that Sema3F-Npn-2 signaling contributes to plasticity in an in vitro model system. Cultured Npn-2 null cortical neurons fail to undergo homeostatic scaling of cell surface AMPA receptors in response to a blockade of excitatory synaptic transmission, supporting that Sema3F- Npn-2/PlexinA3 signaling controls both the development and plasticity of adult cortical circuits. We propose addressing in vivo functions of Sema3F, Npn-2. and PlexinA3 in an adult model of visual cortex plasticity and homeostatic synaptic scaling. We will record from pyramidal neurons in layer 2/3 of adult primary visual cortex in mouse strains following conditional ablation of Sema3F, Npn-2, or PlexinA3. We will also use Cre recombinase driver lines to identify the ligand and receptor-expressing cells that mediate this plasticity. We will use our new HA-epitope tagged Npn-2 knock-in mouse line to immunopurify Npn-2/PlexinA3 complexes from forebrain neurons to identify key intracellular signaling components that mediate AMPA receptor trafficking and plasticity of excitatory synapses. These studies will uncover new functions served by developmentally important neuronal guidance molecules during adult neuronal plasticity and will establish the mechanisms by which semaphorin signaling controls AMPA receptor expression and excitatory neurotransmission.